Pyrazine electron distribution

NH3)5]"+ (n = 4, 5, or 6) and [(bipy)2ClRu(pyz)RuCl(bipy)2]"4 (n = 2, 3, or 4).50 These studies reveal equivalent metal sites even in the mixed-valency Run-Ru,u species, despite the speed with which ESCA monitors electron distribution (10 17 s). Another investigation shows that the complex [Ru(NH3)5(pyz)]2+ can associate with several aqueous first-row transition-metal ions to form pyrazine-bridged dinuclear complexes.51 Equilibrium formation constants with Ni", Cu , and Zn11 were measured spectroscopically. 

ESCA spectra have been recorded for a series of pyrazine-bridged ruthenium dimers including [(NH3)5Ru(pyz)Ru(NH3)5]s H and [(bipy)2ClRu(pyz)RuCl(bipy)2]3+.50 The data show that both Ru and Ruin are present in those complexes although the metal sites are equivalent. The speed with which ESCA monitors electron distribution (ca. 10"17 s) would seem to make it an excellent spectrosocpic tool for such valence-state assignments. 

Electron distribution. The tt- and cr-electron distributions in pyrazine and other azines have been studied theoretically 458,562 there appears to be a reasonable correlation between the net charges on nitrogen atoms and the measured 15N NMR shifts.562... 

Calculations have been made of the rr-electron distribution in the pyrazine ring by many methods (127-134) and the effects of protonation on the total electron densities have been calculated using the extended Hiickel theory (135). A good correlation was obtained between total carbon electron densities and both proton and carbon-13 chemical shifts. A recent molecular orbital study has been made of protonation in pyrazine (and other diazines) (135a). 

The electron distribution varies with the method of calculation but those for pyrazine as determined by Brown and Coller (133) using the VESCF molecular orbital procedure are as shown (22). Albert (136) introduced the term rr-deficient heterocycle to describe such compounds with a deficiency of electrons on carbon. 

The connection between electronic structure and u.v. spectra of pyrazine and protonated pyrazine has been examined the electronic distributions were calculated using a Pariser-Parr-Pople method employing an electrostatic model (proton bounded to the unshared electron pair of the nitrogen atom by electrostatic forces) (1493). The electronic absorption of a number of alkyl-substituted pyrazines in neutral and in acid solvents have also been measured (1476). 

As true reduced flavins , however, and apart from flavohydroquinone, one example of each subclass needs discussion, viz. 4a,5-dihydro-flavin( 5-blocked ) 181,182)and l,8-dihydroflavin( l/2a-blocked ) 12). Both isomers exhibit chromophores which are essentially different from flavohydroquinone, as outlined in Scheme 9. The three dihydro isomers exhibit, furthermore, totally different chemical reactivity 1,8-dihydro-flavin is aromatic with respect to electron distribution in the center (pyrazine) subnucleus, 1,5-dihydroflavin is, contrarily, antiaromatic (unless the pyrazine subnucleus is deplanarized, see below). It contains... 

As discussed in Section III, TRPAD measurements are sensitive to molecular rotational motion by virtue of their geometric dependence on the molecular axis distribution in the LF. An elegant experimental demonstration of this has been performed by Suzuki and co-workers who measured the PAD temporal evolution from excited-state pyrazine [59]. In these experiments, the origin of the Si electronic state of pyrazine was excited by a pump pulse at 323 nm, and... 

The 7r-electron density distribution in the pyrazine ring has been calculated,59-61 and compared with that of the pyridine ring.3 These figures indicate an increase in 7r-electron density at the nitrogen atoms and a depletion of 7r-electron density at the carbon atoms. The values for charge distribution vary slightly with the method of calculation... 

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